Modulators, such as analog-to-digital converters (ADC) or digital modulators, for example, typically produce quantization noise. The quantization noise can cause unwanted contributions in the output spectrum, what are known as spikes, that are sometimes also referred to as idle tones. The quantization noise generally causes a reduction in the quality of a modulator.
Techniques are known in order to reduce such unwanted contributions by the quantization noise e.g. in the form of spikes. These often involve a signal with variable amplitude (dither) being applied in the signal processing of the modulator, e.g. upstream of a quantizer. The amplitude of the dither typically varies on a random or pseudorandom basis. This lowers the signal-to-noise ratio (SNR) of the modulator, since additional noise is imparted in the form of the dither. At the same time, however, the effect achieved is that the quantization noise influences the output spectrum uniformly. This allows the spikes in the output spectrum to be eliminated or reduced, which can be worthwhile in various applications—for example in audio signal processing.
In various reference implementations, dither is produced by using an analog signal generator as a noise source. The analog signal generator can produce an analog random signal that can be spectrally shaped. The analog signal generator can produce a continuous random signal; quantization of the values of the analog random signal typically does not occur. The use of an analog signal generator can be comparatively expensive and complicated, however. The use of an analog signal generator can result in a comparatively high level of hardware complexity.
Therefore, techniques are also known that involve the use of a digital signal generator that produces a digital random signal. Typically, the random signal is pseudorandom with two states, for example ZERO and ONE. Such a scenario is also referred to as 1-bit dither.
A pseudorandom random signal can also be referred to as a pseudorandom signal. The pseudorandom signal can have values that repeat e.g. after a particular number of iterations.
If, in reference implementations, only a single digital signal generator is used, then the dither produced in this way typically does not have particularly good properties for reducing spikes. By way of example, in the case of conventional techniques of this kind that use only a single digital signal generator, the reduction in the SNR can turn out to be particularly severe.
In order to efficiently reduce spikes in the output spectrum of the modulator, a multiplicity of digital 1-bit signal generators is typically used, e.g. two or three or more digital signal generators; this allows what is known as multilevel dither, i.e. having multiple discrete states, e.g. −TWO, −ONE, ZERO, ONE, TWO, or even virtually continuous dither to be produced. Such implementation can have a large number of components, however, and hence be expensive and susceptible to error.